The present invention relates to a novel negative-working deep ultraviolet (UV) photoresist comprising a polymer, photoacid generator and a crosslinking agent. The photoresist is particularly useful for imaging with exposure wavelengths of 193 nanometers (nm) and 157 nm. The invention also relates to a process for imaging the novel photoresist.
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate solvent in the photoresist composition and to fix the coating onto the substrate. The baked, coated surface of the substrate is next subjected to an image-wise exposure to imaging radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are imaging radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the photoresist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to such a solution. Thus, treatment of an exposed negative-working photoresist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating, thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. In a positive-working photoresist the developer removes the portions that are exposed. During the manufacture of devices it is sometimes desirable to use negative-acting photoresists to form images on a substrate.
Photoresist resolution is defined as the smallest feature which the photo composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many leading edge manufacturing applications today, photoresist resolution on the order of less than one-half micron are necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate. This becomes even more critical as the drive toward miniaturization reduces the critical dimensions on the devices.
Photoresists sensitive to short wavelengths, between about 100 nm and about 300 nm are often used where subhalfmicron geometries are required. High resolution, chemically amplified, deep ultraviolet (100-300 nm) positive and negative tone photoresists are available for patterning images with less than quarter micron geometries. To date, there are three major deep ultraviolet (uv) exposure technologies that have provided significant advancement in miniaturization, and these use lasers that emit radiation at 248 nm, 193 nm and 157 nm. Photoresists for 248 nm have typically been based on substituted polyhydroxystyrene and its copolymers, such as those described in U.S. Pat. Nos. 4,491,628 and 5,350,660. On the other hand, photoresists for 193 nm exposure require non-aromatic polymers, since aromatics are opaque at this wavelength. U.S. Pat. No 5,843,624 and GB 232,0718 disclose photoresists useful for 193 nm exposure. Generally, polymers containing alicyclic hydrocarbons are used for photoresists for exposure below 200 nm. Alicyclic hydrocarbons are incorporated into the polymer for many reasons, primarily since they have relatively high carbon:hydrogen ratios which improve etch resistance, they also provide transparency at low wavelengths and they have relatively high glass transition temperatures.
The use of a negative-working, acid-sensitive photoresist composition is known in the prior art. Typically a negative photoresist comprises an alkali-soluble polymer, a photoacid generator and a crosslinking agent. Most of the prior art photoresist compositions use a polymeric binder that is aromatic, such polymers being novolaks or polyhyroxystyrenes. Aromatic polymers, although possessing good dry etch resistance, do not have desirable transparency at wavelengths below 200 nm. Thus there is a need for a negative acting photoresist that is transparent at exposure wavelengths below 200 nm, particularly below 160 nm, and also having good dry etch resistance.
Fluorinated polymers are known for being transparent at 193 nm and 157 nm. Such polymers when used in a photoresist are disclosed in EP 789,278, Ito et al (SPIE Proceedings, Vol. 4345, 2001, pages 273-284), WO 00/67072 and WO 00/17712. WO 00/67072 specifically discloses nonaromatic, alicyclic polymers with pendant fluorinated groups. One such polymer is derived from the polymerization of a norbornene monomer with a pendant xe2x80x94(Rf)(Rfxe2x80x2)ORb group, where Rf and Rfxe2x80x2 are fluoroalkyl groups and Rb is hydrogen or acid-labile group. This polymer is processed with a photoactive compound to give a positive photoresist image. Similarly, Ito describes the possibility of using an all norbornene polymer with pendant hexafluoro-2-hydroxyisopropyl group substituted with an acid labile group to form positive images. Toriumi et al (SPIE Proceedings, Vol. 4345, 2001, pages 371-378) describes a negative photoresist using a fluoropolymer, triphenylsulfonium triflate and a hydroxymethyl compound as a crosslinker, where the photoresist has a sensitivity for a gel dose of 200 mJ/cm2. The structure of the fluoropolymer is not disclosed. Hexafluoro-2-hydroxy-isopropyl groups pendant from styrenic polymers and their use in negative photoresists are described by Przybilla (SPIE Proceedings, Vol. 1672, 1992, pages 500-512). However since these polymers contain aromatic groups they are not useful for imaging at wavelengths below 200 nm.
Thus, there is a need in the semiconductor industry for a negative-acting photoresist that can provide good lithographic properties when imaged at below 200 nm, especially having good bleaching characteristics and good photosensitivity.
The invention pertains to a novel negative-working photoresist that can be developed with an aqueous alkaline solution, and is capable of being imaged at exposure wavelengths below 200 nm. The invention also relates to a process for imaging the novel photoresist. The novel photoresist comprises an alkali soluble fluorinated polymer, a photoacid generator and a crosslinking agent. The polymer has at least one unit of structure 1, 
where Rf1 and Rf2 are independently a perfluorinated or partially fluorinated (C1-C4) alkyl group; and n is 1-8.